Exchangeable or replaceable screen elements of plastics, particularly polyurethane, are known screening media and can have grooves on the underside near the longitudinal sides in order to slide the screen elements with their grooves over metal rails, while an upper region of each metal rail engages in the groove. If a part of the screening unit should be equipped permanently or temporarily with metal screen meshes or screen grills instead of plastic screen elements, plastic bars can be connected by screws with the metal rails so that the metal screens are pressed onto the metal rails. It is necessary for this purpose to drill holes for the screws in the metal screen and in the metal rail. Alternatively it is known, for welded grills, to fasten plastic bars to the metal rails and to provide these plastic bars with horizontal bores in order to slide the ends of the welded grills into the latter. Both modes of fastening are time and labor consuming for assembly and require a particular adaptation for each metal screen. Any change in the types of metal screens necessitates a substantial outlay.